AT&T SYSTEM 25 User Manual page 9

Ringing Tones
System 25 also signals you with four distinctive rings. The rings
and their descriptions are listed below.
(one long ring)
INSIDE CALL
Your call is from a person inside your company.
(two short rings)
OUTSIDE CALL
Your call is from someone outside your company.
(one short burst of ringing)
ABBREVIATED ALERT
This indicates a second call on another line, a busy-to-idle
reminder, a signal, or that your voice terminal is in program
mode.
(two short rings and one
PRIORITY RING
long ring)
Your call is either a callback because the station or outside line
you requested is now available, or your call is being set up
remotely for this phone via Third-Party Call Setup (a data
feature).
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